In recent years, there have been made intensive studies on superconductive materials of oxides which exhibit superconductive phenomena at temperatures higher than those known superconductors composed of materials containing Nb (niobium). This is because if the superconductive phenomena take place at higher temperatures, simpler cooling mechanisms can be used, making it easier to have wide utility of the advantages of such superconductive phenomena in various fields and thus contributing greatly to the development of industries. It has been recently found that Bi-Sr-Ca-Cu-O superconductors exhibit superconductive phenomena at temperatures of higher than 100 K.
When the oxide superconductors including the Bi-Sr-Ca-Cu-O oxides are utilized, for example, in the field of microelectronics such as Josephson element, transistors circuit wirings and the like, it is essential that the superconductor be used as a thin film. Many reports have been made on sputtering for use as a technique of forming such a thin film. However, the sputtering technique is disadvantageous in that when film formation is repeated, there is the tendency that the composition of a target is varied and, thus, the compositional ratio in the thin film does not conform to but greatly differs from the compositional ratio of the target.
For instance, where a superconductive oxide thin film is used as the Josephson element, it is necessary to control the film thickness at a molecular layer level. Such a control of the film thickness is very difficult for the sputtering technique. The length of coherence of the oxide superconductor is so short (e.g. approximately 1 nm) that irregularities of a substrate used to form the thin film or irregularities of the thin film surface itself after the film formation adversely influence the length of coherence. Accordingly, the thin film surface after the film formation should preferably be flat from the standpoint of the molecular layer level. The sputtering technique is difficult in obtaining a thin film with a desired level of flatness.
In order to overcome the above disadvantage, Japanese Laid-open Patent Application No. 64-24319, assigned to the present applicant, proposed formation of a superconductive oxide thin film by a chemical vapor deposition technique. According to this method, the following effects are obtained: (1) to form a super conductive thin film having a very smooth surface and good superconductive characteristics; (2) to give less damages on the substrate or on the formed thin film than those as will be caused by physical film formation methods such as the sputtering technique; (3) to obtain a superconductive thin film with arbitrary composition and crystal structure; (4) to enable one to control the growth of thin film corresponding to the length of coherence.
However, in the prior art techniques including that of Japanese Laid-open Patent Application No. 64-24319, any gas source for bismuth is not particularly described when bismuth-containing oxide superconductive thin films are formed by the chemical vapor deposition technique.
The present inventors made intensive studies primarily on metal complexes containing bismuth in order to obtain a material which is suitable for use as a gas source of bismuth. As a result, it was found that trimethyl bismuth, Bi-(CH.sub.3).sub.3, triethyl bismuth, Bi-(C.sub.2 H.sub.5), or the like which is so reactive as to violently react with air or oxygen at room temperature is unsuitable for use as a gas source for the chemical vapor deposition process. In addition , triacetylacetone bismuth, Bi-(CH.sub.3 COCHCOCH.sub.3).sub.3, or the like is stable at room temperature, but is so low in vapor pressure that when heated, it is decomposed before a satisfactory vapor pressure is obtained, disallowing one to transfer the vapor through a pipe of a chemical vapor deposition apparatus. Thus, this compound is also found unsuitable for use as a gas source for the chemical vapor deposition technique.
It is intended to form a superconductive oxide thin film on a substrate without oxidation wherein if the substrate is made of a material susceptible to oxidation, e.g. a semiconductive substrate such as silicon, GaAs or the like, or a metal electrode, whose electric resistance increases by the oxidation, and a super conductive oxide thin film is further formed on the semiconductive substrate or the metal electrode as a lead wiring, the substrate will be oxidized in an oxygen atmosphere for the film formation by the sputtering or chemical vapor deposition technique, or with a substance containing oxygen used as a gas source used in the chemical vapor deposition technique. Accordingly, there is the demand for a technique which can prevent the oxidation.
Therefore, a first object of the invention is to provide a method for forming a bismuth-containing oxide superconductive thin film by chemical vapor deposition. A second object of the invention is to provide a method for conveniently forming a bismuth-containing oxide superconductive thin film on a substrate susceptible to oxidation by chemical vapor deposition.